As a conventional technique, an electric power supply system disclosed in patent literature 1 is known. When electric power flows back into a commercial power grid, and a voltage of alternating-current power converted by an inverter circuit which connects a solar battery to the commercial power grid exceeds a predetermined value, the electric power supply system starts to operate an electrical load connected between the inverter circuit and the commercial power grid. When the electrical load is operated in this way, self-consumption power increases, so that the amount of backflow of electric power decreases.